The present application relates to interference-suppression means for use in nuclear magnetic resonance (NMR) imaging system and, more particularly, to a novel radio-frequency (RF) shielded room for providing a region, in an NMR imaging system, in which external interference is substantially suppressed.
Nuclear magnetic resonance (NMR) imaging systems, typically capable of scanning any selected portion of the whole human body, utilize relatively large and extremely sensitive RF coils to detect the miniscule NMR response signals which emanate from body tissue. Typically, such RF coils are wound on coil forms that are at least 0.5 meter in diameter. Typical imaging systems require a relatively large static magnetic field, typically in the range from about 0.04 Tesla (T) to about 2.0 T; dependent upon the particular nuclei type being imaged and the static field magnitude, the RF signals will typically occur in the 1-100 MHz. range. This two-decade portion of the electro-magnetic spectrum abounds with relatively high-magnitude RF signals, which tend to interfere with the NMR imaging process. This is particularly true in that the relatively large RF coils and high-sensitivity receivers utilized for NMR imaging are also extremely sensitive to the extraneous RF interference signals, which may radiate, for example, from radio and television stations, electrical machinery, electronic test equipment, computers and the like. Such interference often produces undesirable effects on and/or artifacts in the NMR image. By way of example, an undesired effect can be a generally increased noise level spread throughout the image, resulting in a decrease in image contrast; and an undesired artifact can be a narrow bright streak resulting from a narrow-bandwidth interfering signal which happens to fall within, or even close to, the NMR imaging channel bandwidth.
It is highly desirable, therefore, to provide some means for shielding against the reception of interfering signals during the NMR imaging process. It is also desirable to provide such shielding means at a relatively reasonable cost. While it has been suggested to provide a shielded room about the entire NMR imaging system, the cost of such a shielded room increases generally as the square footage thereof and it is, therefore, desirable to reduce the floor area of a shielded room to be as small as possible. For state-of-the-art NMR imaging systems utilizing a superconducting magnet, a shielded room which contains the entire magnet would have to have a large enough floor area to accommodate the cryogenic dewars which must be brought into reasonably close proximity to the superconducting magnet. Further, a shielded room for use with present superconducting-magnetic NMR imaging systems must be of relatively great height, e.g. typically on the order of 14 feet, to clear the turret of the magnet and provide enough overhead room to allow transfer of cryogenic fluids thereto. A further objection to completely enclosing the NMR imaging system in a shielded room lies in the well-known requirement to shield and/or filter all wiring providing power or interconnecting electrical/electronic equipment within the shielded room to equipment outside of the shielded room, so that these wires and cables do not provide a means for entrance of potentially-interfering signals into the shielded room volume. If the entire magnet is contained within the shielded room, then electrical filters must be installed on each of the leads for: the main magnet; a plurality, typically 3, of imaging-gradient-coil sets; a plurality, typically at least 12, of shim windings which correct the static magnetic field; and other wiring. For example, one particular 1.5 T superconducting magnet requires 13 superconducting shim windings and 10 resistive shim windings, resulting in at least 25 power lines, having no utility in providing the gradient or RF signals, having to be filtered upon entrance through the imaging system shielded room. Each of the multiplicity of required electrical filters is not only expensive, but relatively difficult to design and it is extremely desirable to minimize the number of such filters.